Revisiting the small-world phenomenon: efficiency variation and classification of small-world networks

Research has explored how embeddedness in small-world networks influences individual and firm outcomes. We show that there remains significant heterogeneity among networks classified as small-world networks. We develop measures of the efficiency of a network, which allow us to refine predictions associated with small-world networks. A network is classified as a small-world network if it exhibits a distance between nodes that is comparable to the distance found in random networks of similar sizes—with ties randomly allocated among nodes—in addition to containing dense clusters. To assess how efficient a network is, there are two questions worth asking: (i) ‘what is a compelling random network for baseline levels of distance and clustering?’ and (ii) ‘how proximal should an observed value be to the baseline to be deemed comparable?’. Our framework tests properties of networks, using simulation, to further classify small-world networks according to their efficiency. Our results suggest that small-world networks exhibit significant variation in efficiency. We explore implications for the field of management and organization

Dust detection by the wave instrument on STEREO: nanoparticles picked up by the solar wind?

The STEREO/WAVES instrument has detected a very large number of intense voltage pulses. We suggest that these events are produced by impact ionisation of nanoparticles striking the spacecraft at a velocity of the order of magnitude of the solar wind speed. Nanoparticles, which are half-way between micron-sized dust and atomic ions, have such a large charge-to-mass ratio that the electric field induced by the solar wind magnetic field accelerates them very efficiently. Since the voltage produced by dust impacts increases very fast with speed, such nanoparticles produce signals as high as do much larger grains of smaller speeds. The flux of 10-nm radius grains inferred in this way is compatible with the interplanetary dust flux model. The present results may represent the first detection of fast nanoparticles in interplanetary space near Earth orbit.Comment: In press in Solar Physics, 13 pages, 5 figure

Are we seeing accretion flows in a 250kpc-sized Ly-alpha halo at z=3?

Using MUSE on the ESO-VLT, we obtained a 4 hour exposure of the z=3.12 radio galaxy MRC0316-257. We detect features down to ~10^-19 erg/s/cm^2/arcsec^2 with the highest surface brightness regions reaching more than a factor of 100 higher. We find Ly-alpha emission out to ~250 kpc in projection from the active galactic nucleus (AGN). The emission shows arc-like morphologies arising at 150-250 kpc from the nucleus in projection with the connected filamentary structures reaching down into the circum-nuclear region. The most distant arc is offset by 700 km/s relative to circum-nuclear HeII 1640 emission, which we assume to be at the systemic velocity. As we probe emission closer to the nucleus, the filamentary emission narrows in projection on the sky, the relative velocity decreases to ~250 km/s, and line full-width at half maximum range from 300-700 km/s. From UV line ratios, the emission on scales of 10s of kpc from the nucleus along a wide angle in the direction of the radio jets is clearly excited by the radio jets and ionizing radiation of the AGN. Assuming ionization equilibrium, the more extended emission outside of the axis of the jet direction would require 100% or more illumination to explain the observed surface brightness. High speed (>300 km/s) shocks into rare gas would provide sufficiently high surface brightness. We discuss the possibility that the arcs of Ly-alpha emission represent accretion shocks and the filamentary emission represent gas flows into the halo, and compare our results with gas accretion simulations.Comment: 4 pages, 2 figures, 1 table, A&A letters accepte

Galaxy protocluster candidates around z ~ 2.4 radio galaxies

We study the environments of 6 radio galaxies at 2.2 < z < 2.6 using wide-field near-infrared images. We use colour cuts to identify galaxies in this redshift range, and find that three of the radio galaxies are surrounded by significant surface overdensities of such galaxies. The excess galaxies that comprise these overdensities are strongly clustered, suggesting they are physically associated. The colour distribution of the galaxies responsible for the overdensity are consistent with those of galaxies that lie within a narrow redshift range at z ~ 2.4. Thus the excess galaxies are consistent with being companions of the radio galaxies. The overdensities have estimated masses in excess of 10^14 solar masses, and are dense enough to collapse into virizalised structures by the present day: these structures may evolve into groups or clusters of galaxies. A flux-limited sample of protocluster galaxies with K < 20.6 mag is derived by statistically subtracting the fore- and background galaxies. The colour distribution of the protocluster galaxies is bimodal, consisting of a dominant blue sequence, comprising 77 +/- 10% of the galaxies, and a poorly populated red sequence. The blue protocluster galaxies have similar colours to local star-forming irregular galaxies (U -V ~ 0.6), suggesting most protocluster galaxies are still forming stars at the observed epoch. The blue colours and lack of a dominant protocluster red sequence implies that these cluster galaxies form the bulk of their stars at z < 3.Comment: Accepted for publication in MNRA

Dependence of geosynchrotron radio emission on the energy and depth of maximum of cosmic ray showers

Based on CORSIKA and REAS2 simulations, we investigate the dependence of geosynchrotron radio emission from extensive air showers on the energy of the primary cosmic ray and the depth of the shower maximum. It is found that at a characteristic lateral distance, the amplitude of the bandpass-filtered radio signal is directly proportional to the energy deposited in the atmosphere by the electromagnetic cascade, with an RMS uncertainty due to shower-to-shower fluctuations of less than 3%. In addition, the ratio of this radio amplitude and that at a larger lateral distance is directly related to the atmospheric depth of the shower maximum, with an RMS uncertainty of ~15-20 g cm-2. By measuring these quantities, geosynchrotron radio emission from cosmic ray air showers can be used to infer the energy of the primary particle and the depth of the air shower maximum on a shower-to-shower basis.Comment: version accepted by Astroparticle Physics; slightly changed title and wording; one additional figur

The Inactivation of a New Peptidoglycan Hydrolase Pmp23 Leads to Abnormal Septum Formation in Streptococcus pneumoniae

The bacterial peptidoglycan is the major component of the cell wall which integrity is essential to cell survival. In a previous work, we identified, in the positive-Gram pathogen Streptococcus pneumoniae , a unique protein containing a new putative peptidoglycan hydrolytic domain named PECACE (PEptidoglycan CArbohydrate Cleavage Enzyme). In this study, we characterise the physiological function of this protein called Pmp23 (Pneumococcal Membrane Protein of 23 kDa). A cell wall hydrolytic activity is observed with the recombinant protein. Inactivation of the pmp23 gene in the pneumococcus led to a decreased flocculation, an increased sensitivity to β-lactam antibiotics and morphological alterations affecting the formation and localisation of the division septa. Taken together these observations indicate that Pmp23 is a hydrolase whose function is linked to peptidoglycan metabolism at the septum site

Near-IR bright galaxies at z~2. Entering the spheroid formation epoch ?

Spectroscopic redshifts have been measured for 9 K-band luminous galaxies at 1.7 < z < 2.3, selected with Ks < 20 in the "K20 survey" region of the Great Observatories Origins Deep Survey area. Star formation rates (SFRs) of ~100-500 Msun/yr are derived when dust extinction is taken into account. The fitting of their multi-color spectral energy distributions indicates stellar masses M ~ 10^11 Msun for most of the galaxies. Their rest-frame UV morphology is highly irregular, suggesting that merging-driven starbursts are going on in these galaxies. Morphologies tend to be more compact in the near-IR, a hint for the possible presence of older stellar populations. Such galaxies are strongly clustered, with 7 out of 9 belonging to redshift spikes, which indicates a correlation length r_0 ~ 9-17 h^-1 Mpc (1 sigma range). Current semianalytical models of galaxy formation appear to underpredict by a large factor (about 30) the number density of such a population of massive and powerful starburst galaxies at z ~ 2. The high masses and SFRs together with the strong clustering suggest that at z ~ 2 we may have started to explore the major formation epoch of massive early-type galaxies.Comment: accepted on June 17. To appear on ApJ Letter

The First Appearance of the Red Sequence of Galaxies in Proto-Clusters at 2<~z<~3

We explore the evolved galaxy population in the proto-clusters around four high-z radio galaxies at 2<~z<~3 based on wide-field near-infrared imaging. Three of the four fields are known proto-clusters as demonstrated by overdensities of line emitting galaxies at the same redshifts as the radio galaxies. We imaged the fields of three targets (PKS1138-262, USS0943-242 and MRC0316-257) to a depth of Ks~22 (5sigma) over a 4'x7' area centered on the radio galaxies with a new wide-field NIR camera, MOIRCS, on the Subaru Telescope. Another target (USS1558-003) was observed with SOFI on the NTT to a depth of Ks=20.5 over a 5'x5' area. We apply colour cuts in J-Ks and/or JHKs in order to exclusively search for galaxies located at high redshifts: z>2. To the 5sigma limiting magnitudes, we see a significant excess of NIR selected galaxies by a factor of two to three compared to those found in the field of GOODS-South. The spatial distribution of these NIR selected galaxies is not uniform and traces structures similar to those of emission line galaxies, although the samples of NIR selected galaxies and emitters show little overlap. We focus on the NIR colour-magnitude sequence of the evolved population and find that the bright-end (M_{stars}>10^{11}Msun) of the red sequence is well populated by z~2 but much less so in the z~3 proto-clusters. This may imply that the bright-end of the colour-magnitude sequence first appeared between z=3 and 2, an era coinciding with the appearance of submm galaxies and the peak of the cosmic star formation rate. Our observations show that during the same epoch, massive galaxies are forming in high density environments by vigorous star formation and assembly.Comment: Accepted for publication in MNRAS, 10 pages, 16 postscript figures, uses mn2e.cl

Strong [CII] emission at high redshift

We report the detection of the [CII]157.74um fine-structure line in the lensed galaxy BRI 0952-0115 at z=4.43, using the APEX telescope. This is the first detection of the [CII] line in a source with L_FIR < 10^13 L_sun at high redshift. The line is very strong compared to previous [CII] detections at high-z (a factor of 5-8 higher in flux), partly due to the lensing amplification. The L_[CII]/L_FIR ratio is 10^-2.9, which is higher than observed in local galaxies with similar infrared luminosities. Together with previous observations of [CII] at high redshift, our result suggests that the [CII] emission in high redshift galaxies is enhanced relative to local galaxies of the same infrared luminosity. This finding may result from selection effects of the few current observations of [CII] at high redshift, and in particular the fact that non detections may have not been published (although the few published upper limits are still consistent with the [CII] enhancement scenario). If the trend is confirmed with larger samples, it would indicate that high-z galaxies are characterized by different physical conditions with respect to their local counterparts. Regardless of the physical origin of the trend, this effect would increase the potential of the [CII]158um line to search and characterize high-z sources.Comment: Accepted for publication in A&A Letters, 5 pages, 2 figure